If you weren't drinking the beer, and you weren't going to open one in front of anyone that night then you would have been perfectly ok saying something like, "Oh, I'm sorry. I meant to put those away. Those are a gift for Fred. We have Brand x, y, or Z for the party- what would you like?" Then get them drinks and put the beer you like away (You could have quietly poured one in a glass for yourself later). If the person is horribly rude enough to beg for one anyway, simply repeat- "I'm sorry, you must not have heard me. Those are a not for the party. Now would you like X,Y, or Z?"
If, however, you had thoughts of drinking the beer yourself, openly, and simply telling the guests "No, this is for me not you. I went to a lot of trouble to get it," then it's straight to ehell you go, unfortunately. This is the same thing as inviting people to dinner and serving yourselves steak and them hamburger- because you were the hosts, and did buy beer for guests (even though it was BYOB) you cannot reserve the "best" beer for yourselves (at least not visibly). If you weren't providing beer for anyone then you would have been marginally okay saying no, but come on. I know amongst my group of friends we do a lot of BYOB, and someone always wants to taste someone else's beer. Those who don't want to lose 5 of 6 bottles of their precious special beverage don't bring it.
When stopping in the next day, you said your FIL passed around what was left of the beer- serving people who were guests in your home. It is the host's responsibility to offer guests food/drinks (or not)- not anyone elses. You would have been perfectly fine to say "I'm sorry, we are not drinking those today. We do have X, Y, and Z- what would everyone like?" Again- it would not be fine for you to sit drinking a "special" beer and telling people "No, these are not for you."